1. Field of the Invention
The present invention relates to a communication apparatus having function of generating a calling signal.
2. Related Background Art
Hitherto, there is known a facsimile apparatus having functions such as actuation of an automatic-answer/record phone and ringing of a telephone set.
According to such a facsimile apparatus, a calling signal generation circuit is arranged on a printed circuit board of an NCU (Network Control Unit) or an independent printed circuit board. A calling signal is sent from the circuit to perform an operation of actuation of the auto-answer/record phone and ringing of the telephone set as described above.
A conventional calling signal generation circuit employs a DC-DC converter system as a source voltage in order to obtain a high voltage (e.g., +24 V to -150 V) as calling signal. The circuit is so arranged that a boosted voltage obtained at the secondary end is controlled by a control signal from a main body circuit of the facsimile apparatus.
The conventional apparatus, however, has been associated with the following drawbacks, since the calling signal generation circuit having a DC--DC converter is incorporated in an NCU board, or arranged on an independent circuit board.
First, as mentioned above, since the calling signal is given with a high voltage (e.g., +24 V to --150 V), it is deemed on the safety standard as a dangerous voltage, and thus it is necessary for a secondary circuit of the DC--DC converter to be provided with a predetermined withstand voltage determined by the safety standard. However, in order to provide such a withstand voltage, it is obliged that an oscillation transformer of the DC-DC converter withstands a high voltage, and in addition, a control signal for controlling the calling signal is insulatingly supplied from the main body circuit to the calling signal generation circuit by using a photo-coupler or the like.
Second, while an oscillation frequency of the DC--DC converter is of the order of 10 KHz plus several KHz, its higher harmonic wave invades as a noise to other circuits. It is not easy to remove such a noise.
As to a calling signal generation circuit, there is known U.S. Pat. No. 4,942,600 (issued Jul. 17, 1990). As to a facsimile apparatus having a calling signal generation circuit, there are U.S. patent application Ser. Nos. 829,592 (filed Feb. 3, 1991) and 702,388 (filed May 20, 1991). However, there has been no proposal for removing the above-mentioned problems.